Integrated endpoint detection system with optical and eddy current monitoring

ABSTRACT

A chemical mechanical polishing apparatus and method can use an eddy current monitoring system and an optical monitoring system. Signals from the monitoring systems can be combined on an output line and extracted by a computer. A thickness of a polishing pad can be calculated. The eddy current monitoring system and optical monitoring system can measure substantially the same location on the substrate.

BACKGROUND

The present invention relates generally to chemical mechanical polishingof substrates, and more particularly to methods and apparatus formonitoring a metal layer during chemical mechanical polishing.

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon wafer. One fabrication step involves depositing a fillerlayer over a non-planar surface, and planarizing the filler layer untilthe non-planar surface is exposed. For example, a conductive fillerlayer can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. The filler layer is thenpolished until the raised pattern of the insulative layer is exposed.After planarization, the portions of the conductive layer remainingbetween the raised pattern of the insulative layer form vias, plugs andlines that provide conductive paths between thin film circuits on thesubstrate. In addition, planarization is needed to planarize thesubstrate surface for photolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a rotating polishing disk pad or beltpad. The polishing pad can be either a “standard” pad or afixed-abrasive pad. A standard pad has a durable roughened surface,whereas a fixed-abrasive pad has abrasive particles held in acontainment media. The carrier head provides a controllable load on thesubstrate to push it against the polishing pad. A polishing slurry,including at least one chemically-reactive agent, and abrasive particlesif a standard pad is used, is supplied to the surface of the polishingpad.

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness, or when a desired amount of material hasbeen removed. Overpolishing (removing too much) of a conductive layer orfilm leads to increased circuit resistance. On the other hand,under-polishing (removing too little) of a conductive layer leads toelectrical shorting. Variations in the initial thickness of thesubstrate layer, the slurry composition, the polishing pad condition,the relative speed between the polishing pad and the substrate, and theload on the substrate can cause variations in the material removal rate.These variations cause variations in the time needed to reach thepolishing endpoint. Therefore, the polishing endpoint cannot bedetermined merely as a function of polishing time.

One way to determine the polishing endpoint is to remove the substratefrom the polishing surface and examine it. For example, the substratecan be transferred to a metrology station where the thickness of asubstrate layer is measured, e.g., with a profilometer or a resistivitymeasurement. If the desired specifications are not met, the substrate isreloaded into the CMP apparatus for further processing. This is atime-consuming procedure that reduces the throughput of the CMPapparatus. Alternatively, the examination might reveal that an excessiveamount of material has been removed, rendering the substrate unusable.

More recently, in-situ monitoring of the substrate has been performed,e.g., with optical or capacitance sensors, in order to detect thepolishing endpoint. Other proposed endpoint detection techniques haveinvolved measurements of friction, motor current, slurry chemistry,acoustics and conductivity. One detection technique that has beenconsidered is to induce an eddy current in the metal layer and measurethe change in the eddy current as the metal layer is removed.

SUMMARY

In one aspect, the invention is directed to an apparatus for chemicalmechanical polishing. The apparatus has a platen to support a polishingsurface, an eddy current monitoring system positioned in the platen togenerate a first signal, an optical monitoring system positioned in theplaten to generate a second signal, circuitry in the platen to combinethe first and second signals into a third signal on an output line, anda computer to receive the third signal on the output line and extractthe first and second signals.

Implementations of the invention may include one or more of thefollowing features. The platen may be rotatable, and the output line maypass through a rotary electrical union between the circuitry and thecomputer. A carrier head may hold a substrate in contact with thepolishing surface. The circuitry may assemble data from the first andsecond signals into packets, and the computer may extract the data fromthe packets.

In another aspect, the invention is directed to a method of determiningthe thickness of a polishing pad. In the method, a substrate having aconductive layer disposed thereon is positioned in contact with apolishing surface of a polishing pad. An alternating magnetic field isgenerated from an inductor to induce eddy currents in the conductivelayer. A strength of the magnetic field is measured, and a thickness ofthe polishing pad is calculated from at least the strength of themagnetic field.

Implementations of the invention may include one or more of thefollowing features. Generating the alternating magnetic field mayinclude driving the inductor with a drive signal. A phase differencebetween the magnetic field and the drive signal may be measured. Thethickness of the polishing pad may be calculated from at least thestrength of the magnetic field and the phase difference. A testsubstrate may be polished with a first polishing pad having a firstknown thickness and with a second polishing pad having a second knownthickness, and at least one coefficient may be generated to relate thethickness of the polishing pad to the strength of the signal duringpolishing. A user may be alerted if the thickness of the polishing padfalls below a predetermined thickness.

In another aspect, the invention is directed to a method of measuring athickness of a conductive layer on a substrate during chemicalmechanical polishing. In the method, a substrate having a conductivelayer disposed thereon is positioned in contact with a polishing surfaceof a polishing pad. Relative motion is created between the substrate andthe polishing pad to polish the substrate. An inductor is driven with adrive signal to generate an alternating magnetic field that induces eddycurrents in the conductive layer, a strength of the magnetic field and aphase difference between the magnetic field and the drive signal aremeasured, a correction factor is calculated based on the strength of themagnetic field, and a thickness of the conductive layer is calculatedfrom the phase difference and the correction factor.

Implementations of the invention may include one or more of thefollowing features. A thickness of the polishing pad may be calculatedfrom at least the strength of the magnetic field. A test substrate maybe polished with a first polishing pad having a first known thicknessand with a second polishing pad having a second known thickness, or atest substrate may be polished with a first polishing pad when the firstpolishing pad has a first known thickness and polished with the firstpolishing pad when the first polishing pad has a second known thickness.At least one coefficient may be generated to relate the thickness of thepolishing pad to the strength of the signal during polishing. A user maybe alerted if the thickness of the polishing pad falls below apredetermined thickness.

In another aspect, the invention is directed to a chemical mechanicalpolishing apparatus. The apparatus has a polishing surface, a carrierhead to hold a substrate having a conductive layer disposed thereon incontact with the polishing surface, a motor to create relative motionbetween the substrate and the polishing surface, an eddy currentmonitoring system including an inductor and a current source to drivethe inductor to generate an alternating magnetic field that induces eddycurrents in the conductive layer, a sensor to measure a strength of themagnetic field and a phase difference between the magnetic field and thedrive signal, and a computer configured to calculate a correction factorbased on the strength of the magnetic field and calculate a thickness ofthe conductive layer from the phase difference and the correctionfactor.

In another aspect, the invention is directed to an apparatus forchemical mechanical polishing. The apparatus has a platen to support apolishing surface, a carrier head to hold a substrate, an eddy currentmonitoring system to generate a first signal during polishing, and anoptical monitoring system positioned to generate a second signal duringpolishing. The eddy current monitoring system includes an inductor togenerate a magnetic field that extends to a first region of thesubstrate, and the optical monitoring system includes a light sourcepositioned and oriented to direct a light beam to a spot in the firstregion of the substrate. Thus, the eddy current monitoring system andoptical monitoring system measure substantially the same location on thesubstrate.

Implementations of the invention may include one or more of thefollowing features. The eddy current monitoring system may include acore having a plurality of prongs. The optical monitoring system mayincludes a detector positioned at least partially between the prongs.The light beam may impinge the substrate at a point substantiallyequidistant from the prongs. The light beam may impinge the substrate ata spot directly above the core.

Possible advantages of implementations of the invention can include oneor more of the following. The optical and eddy current monitoringsystems can monitor essentially the same spot on the substrate. Thethickness of the conductive layer can be measured during bulk polishing.The thickness of a polishing pad used to polish the substrate can alsobe measured during polishing. The pressure profile applied by thecarrier head can be adjusted to compensate for non-uniform polishingrates and non-uniform thickness of the incoming substrate. Polishing canbe stopped with high accuracy. Over-polishing and under-polishing can bereduced, as can dishing and erosion, thereby improving yield andthroughput.

Other features and advantages of the invention will become apparent fromthe following description, including the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic exploded perspective view of a chemical mechanicalpolishing apparatus.

FIG. 2 is a schematic cross-sectional view of a carrier head.

FIG. 3A is a schematic side view, partially cross-sectional, of achemical mechanical polishing station that includes an eddy currentmonitoring system and an optical monitoring system.

FIG. 3B is a schematic top view of a platen from the polishing stationof FIG. 3A.

FIG. 4 is a schematic circuit diagram of the eddy current monitoringsystem.

FIG. 5 is a schematic cross-sectional view illustrating a magnetic fieldgenerated by the eddy current monitoring system.

FIGS. 6A-6D schematically illustrate a method of detecting a polishingendpoint using an eddy current sensor.

FIG. 7 is a graph illustrating an amplitude trace from the eddy currentmonitoring system.

FIG. 8 is a schematic circuit diagram of an eddy current monitoringsystem that senses an amplitude and a phase shift.

FIG. 9 is a graph illustrating a phase shift trace from the eddy currentmonitoring system.

FIGS. 10A-10C are cross-sectional views of a platen with an optical andeddy current monitoring system.

FIG. 11 is a graph illustrating an amplitude trace from the opticalmonitoring system.

FIGS. 12A and 12B are graphs illustrating amplitude traces and phasedifference traces, respectively, generated by the eddy currentmonitoring system at different pad thicknesses.

DETAILED DESCRIPTION

Referring to FIGS. 1, one or more substrates 10 can be polished by a CMPapparatus 20. A description of a similar polishing apparatus 20 can befound in U.S. Pat. No. 5,738,574, the entire disclosure of which isincorporated herein by reference. Polishing apparatus 20 includes aseries of polishing stations 22 a, 22 b and 22 c, and a transfer station23.

Each polishing station includes a rotatable platen 24 on which is placeda polishing pad 30. The first and second stations 22 a and 22 b caninclude a two-layer polishing pad with a hard durable outer surface or afixed-abrasive pad with embedded abrasive particles. The final polishingstation 22 c can include a relatively soft pad or a two-layer pad. Eachpolishing station can also include a pad conditioner apparatus 28 tomaintain the condition of the polishing pad so that it will effectivelypolish substrates.

Referring to FIG. 3A, a two-layer polishing pad 30 typically has abacking layer 32 which abuts the surface of platen 24 and a coveringlayer 34 which is used to polish substrate 10. Covering layer 34 istypically harder than backing layer 32. However, some pads have only acovering layer and no backing layer. Covering layer 34 can be composedof foamed or cast polyurethane, possibly with fillers, e.g., hollowmicrospheres, and/or a grooved surface. Backing layer 32 can be composedof compressed felt fibers leached with urethane. A two-layer polishingpad, with the covering layer composed of IC-1000 and the backing layercomposed of SUBA-4, is available from Rodel, Inc., of Newark, Del.(IC-1000 and SUBA-are product names of Rodel, Inc.).

During a polishing step, a slurry 38 containing a liquid (e.g.,deionized water for oxide polishing) and a pH adjuster (e.g., potassiumhydroxide for oxide polishing) can be supplied to the surface ofpolishing pad 30 by a slurry supply port or combined slurry/rinse arm39. If polishing pad 30 is a standard pad, slurry 38 can also includeabrasive particles (e.g., silicon dioxide for oxide polishing).

Returning to FIG. 1, a rotatable multi-head carousel 60 supports fourcarrier heads 70. The carousel is rotated by a central post 62 about acarousel axis 64 by a carousel motor assembly (not shown) to orbit thecarrier head systems and the substrates attached thereto betweenpolishing stations 22 and transfer station 23. Three of the carrier headsystems receive and hold substrates, and polish them by pressing themagainst the polishing pads. Meanwhile, one of the carrier head systemsreceives a substrate from and delivers a substrate to a loadingapparatus via transfer station 23.

Each carrier head 70 is connected by a carrier drive shaft 74 to acarrier head rotation motor 76 (shown by the removal of one quarter ofcover 68) so that each carrier head can independently rotate about itown axis. In addition, each carrier head 70 independently laterallyoscillates in a radial slot 72 formed in carousel support plate 66. Adescription of a suitable carrier head 70 can be found in U.S. patentapplication Ser. Nos. 09/470,820 and 09/535,575, filed Dec. 23, 1999 andMar. 27, 2000, the entire disclosures of which are incorporated byreference. In operation, the platen is rotated about its central axis25, and the carrier head is rotated about its central axis 71 andtranslated laterally across the surface of the polishing pad.

As disclosed in the foregoing patent applications and as shown in FIG.2, an exemplary carrier head 70 includes a housing 202, a base assembly204, a gimbal mechanism 206 (which can be considered part of the baseassembly 204), a loading chamber 208, a retaining ring 210, and asubstrate backing assembly 212 which includes three pressurizablechambers, such as a floating upper chamber 236, a floating lower chamber234, and an outer chamber 238. Loading chamber 208 is located betweenhousing 202 and base assembly 204 to apply a load to and to control thevertical position of base assembly 204. A first pressure regulator (notshown) can be fluidly connected to loading chamber 208 by a passage 232to control the pressure in the loading chamber and the vertical positionof base assembly 204.

The substrate backing assembly 212 includes a flexible internal membrane216, a flexible external membrane 218, an internal support structure220, an external support structure 230, an internal spacer ring 222 andan external spacer ring 232. Flexible internal membrane 216 includes acentral portion which applies pressure to substrate 10 in a controllablearea. The volume between base assembly 204 and internal membrane 216that is sealed by an inner flap 244 provides pressurizable floatinglower chamber 234. The annular volume between base assembly 204 andinternal membrane 216 that is sealed by inner flap 244 and outer flap246 defines pressurizable floating upper chamber 236. The sealed volumebetween internal membrane 216 and external membrane 218 defines apressurizable outer chamber 238. Three pressure regulators (not shown)can be independently connected to floating lower chamber 234, floatingupper chamber 236 and outer chamber 238. Thus, a fluid such as a gas canbe directed into or out of each chamber independently.

The combination of pressures in floating upper chamber 236, floatinglower chamber 234 and outer chamber 238 control both the contact areaand the pressure of internal membrane 216 against a top surface of theexternal membrane 218. For example, by pumping fluid out of floatingupper chamber 236, the edge of internal membrane 216 is lifted away fromexternal membrane 218, thereby decreasing the contact diameter DC of thecontact area between the internal membrane and external membrane.Conversely, by pumping fluid into floating upper chamber 236, the edgeof internal membrane 216 is lowered toward external membrane 218,thereby increasing the contact diameter DC of the contact area. Inaddition, by pumping fluid into or out of floating lower chamber 234,the pressure of internal membrane 216 against external membrane 218 canbe varied. Thus, the pressure in and the diameter of the area loaded bythe carrier head can be controlled.

Referring to FIGS. 3A and 3B, a recess 26 is formed in platen 24, and atransparent cover 27, e.g., of glass or a hard plastic, can be placedover recess 26. In addition, a transparent section 36 is formed inpolishing pad 30 overlying transparent cover 27. Transparent cover 27and transparent section 36 are positioned such that they pass beneathsubstrate 10 during a portion of the platen's rotation, regardless ofthe translational position of the carrier head. Assuming that polishingpad 32 is a two-layer pad, transparent section 36 can be constructed bycutting an aperture in backing layer 32, and by replacing a section ofcover layer 34 with a transparent plug. The plug can be a relativelypure polymer or polyurethane, e.g., formed without fillers. In general,the material of transparent section 36 should be non-magnetic andnon-conductive.

Referring to FIG. 3A, the first polishing station 22 a includes anin-situ eddy current monitoring system 40 and an optical monitoringsystem 140. The eddy current monitoring system 40 and optical monitoringsystem 140 can function as a polishing process control and endpointdetection system. The second polishing station 22 b and the finalpolishing station 22 c can both include just an optical monitoringsystem, although either may additionally include an eddy currentmonitoring system or only an eddy current monitoring system.

As shown by FIG. 3B, core 42 and window section 36 sweeps beneath thesubstrate 10 with each rotation of the platen. Each time the windowsection sweeps beneath the substrate, data can be collected from eddycurrent monitoring system 40 and optical monitoring system 140.

Referring to FIG. 4, eddy current monitoring system 40 includes a drivesystem 48 to induce eddy currents in a metal layer on the substrate anda sensing system 58 to detect eddy currents induced in the metal layerby the drive system. The monitoring system 40 includes a core 42positioned in recess 26 to rotate with the platen, a drive coil 44 woundaround one part of core 42, and a sense coil 46 wound around second partof core 42. For drive system 48, monitoring system 40 includes anoscillator 50 connected to drive coil 44. For sense system 58,monitoring system 40 includes a capacitor 52 connected in parallel withsense coil 46, an RF amplifier 54 connected to sense coil 46, and adiode 56. The oscillator 50, capacitor 52, RF amplifier 54, and diode 56can be located on a printed circuit board 160 inside the recess 26. Acomputer 90 can be coupled to the components in the platen, includingprinted circuit board 160, through a rotary electrical union 92.

Referring to FIG. 5, core 42 can be a U-shaped body formed of anon-conductive material with a relatively high magnetic permeability.The driving coil can be designed to match the driving signal from theoscillator. The exact winding configuration, core composition and shape,and capacitor size can be determined experimentally. As shown, the lowersurface of transparent cover 27 may include two rectangular indentations29, and the two prongs 42 a and 42 b of core 42 may extend into theindentations so as to be positioned closer to the substrate.

Returning to FIG. 3A, in operation, oscillator 50 drives drive coil 44to generate an oscillating magnetic field 48 that extends through thebody of core 42 and into the gap 46 between the two poles 42 a and 42 bof the core. At least a portion of magnetic field 48 extends throughthin portion 36 of polishing pad 30 and into substrate 10. If a metallayer 12 is present on substrate 10, oscillating magnetic field 48generates eddy currents in the metal layer 12. The eddy currents causethe metal layer 12 to act as an impedance source in parallel with sensecoil 46 and capacitor 52. As the thickness of the metal layer changes,the impedance changes, resulting in a change in the Q-factor of sensingmechanism. By detecting the change in the Q-factor of the sensingmechanism, the eddy current sensor can sense the change in the strengthof the eddy currents, and thus the change in thickness of metal layer12.

In general, the greater the expected initial thickness of the conductivefilm, the lower the desired resonant frequency. For example, for arelatively thin film, e.g., 2000 Angstroms, the capacitance andinductance can be selected to provide a relatively high resonantfrequency, e.g., about 2 MHz. On the other hand, for a relativelythicker film, e.g., 20000 Angstroms, the capacitance and inductance canbe selected to provide a relatively lower resonant frequency, e.g.,about 50 kHz. However, high resonant frequencies may still work wellwith thick copper layers. In addition, very high frequencies (above 2MHz) can be used to reduce background noise from metal parts in thecarrier head.

Initially, referring to FIGS. 3A, 4 and 6A, before conducting polishing,oscillator 50 is tuned to the resonant frequency of the LC circuit,without any substrate present. This resonant frequency results in themaximum amplitude of the output signal from RF amplifier 54.

As shown in FIGS. 6B and 7, for a polishing operation, a substrate 10 isplaced in contact with polishing pad 30. Substrate 10 can include asilicon wafer 12 and a conductive layer 16, e.g., a metal such ascopper, disposed over one or more patterned underlying layers 14, whichcan be semiconductor, conductor or insulator layers. A barrier layer 18,such as tantalum or tantalum nitride, may separate the metal layer fromthe underlying dielectric.

The patterned underlying layers can include metal features, e.g., vias,pads and interconnects. Since, prior to polishing, the bulk ofconductive layer 16 is initially relatively thick and continuous, it hasa low resistivity, and relatively strong eddy currents can be generatedin the conductive layer. As previously mentioned, the eddy currentscause the metal layer to function as an impedance source in parallelwith sense coil 46 and capacitor 52. Consequently, the presence ofconductive film 16 reduces the Q-factor of the sensor circuit, therebysignificantly reducing the amplitude of the signal from RF amplifier 56.

Referring to FIGS. 6C and 7, as substrate 10 is polished, the bulkportion of conductive layer 16 is thinned. As conductive layer 16 thins,its sheet resistivity increases, and the eddy currents in the metallayer become dampened. Consequently, the coupling between metal layer 16and sensor circuitry 58 is reduced (i.e., increasing the resistivity ofthe virtual impedance source). As the coupling declines, the Q-factor ofthe sensor circuit 58 increases toward its original value.

Referring to FIGS. 6D and 7, eventually the bulk portion of conductivelayer 16 is removed, leaving conductive interconnects 16′ in thetrenches between the patterned insulative layer 14. At this points, thecoupling between the conductive portions in the substrate, which aregenerally small and generally non-continuous, and sensor circuit 58reaches a minimum. Consequently, the Q-factor of the sensor circuitreaches a maximum value (although not as large as the Q-factor when thesubstrate is entirely absent). This causes the amplitude of the outputsignal from the sensor circuit to plateau.

In addition to sensing changes in amplitude, the eddy current monitoringsystem can calculate a phase shift in the sensed signal. As the metallayer is polished, the phase of the sensed signal changes relative tothe drive signal from oscillator 50. This phase difference can becorrelated to the thickness of the polished layer.

One implementation of a phase measuring device, shown in FIG. 8,combines the drive and sense signals to generate both an amplitudesignal and a phase shift signal with a pulse width or duty cycle whichis proportional to the phase difference. In this implementation, two XORgates 100 and 102 are used to convert sinusoidal signals from sense coil46 and oscillator 50, respectively, into square-wave signals. The twosquare-wave signals are fed into the inputs of a third XOR gate 104. Theoutput of the third XOR gate 104 is a phase shift signal with a pulsewidth or duty cycle proportional to the phase difference between the twosquare wave signals. The phase shift signal is filtered by an RC filter106 to generate a DC-like signal with a voltage proportional to thephase difference. Alternatively, the signals can be fed into aprogrammable digital logic, e.g., a Complex Programmable Logic Device(CPLD) or Field Programmable Gate Array (FGPA) that performs the phaseshift measurements. An example of a trace generated by the eddy currentmonitoring system that measures the phase difference between the driveand sense signals is shown in FIG. 9. Since the phase measurements arehighly sensitive to the stability of the driving frequency, phase lockedloop electronics may be added.

A possible advantage of the phase difference measurement is that thedependence of the phase difference on the metal layer thickness may bemore linear than that of the amplitude. In addition, the absolutethickness of the metal layer may be determined over a wide range ofpossible thicknesses.

Returning to FIG. 3A, optical monitoring system 140, which can functionas a reflectometer or interferometer, can be secured to platen 24 inrecess 26 with eddy current monitoring system 40. The optical monitoringsystem 140 includes a light source 144 and a detector 146. Theelectronics for light source 144 and detector 146 may be located on

printed circuit board 160. The light source generates a light beam 142which propagates through transparent window section 36 and slurry toimpinge upon the exposed surface of the substrate 10. For example, lightsource 144 may be a laser and light beam 142 may be a collimated laserbeam. Light laser beam 142 can be projected from laser 144 at an anglefrom an axis normal to the surface of substrate 10. In addition, if hole26 and window 36 are elongated, a beam expander (not illustrated) may bepositioned in the path of the light beam to expand the light beam alongthe elongated axis of the window. In general, the optical monitoringsystem functions as described in U.S. patent application Ser. Nos.09/184,775, filed Nov. 2, 1998, and 09/184,767, filed Nov. 2, 1998, theentire disclosures of which are incorporated herein by references.

Referring to FIGS. 10A-10C, optical monitoring system 140 can bepositioned so that light beam 142 impinges the substrate at a positionbetween two prongs 43 of core 42. In one implementation, light source144 is positioned to direct light beam 142 toward core 42 along a pathsubstantially parallel to the surface of platen 24. The light beam 142is reflected upwardly from a mirror 162 positioned just before core 42so that light beam 142 passes between prongs 43, is reflected fromsubstrate 10, and then impinges a detector 146 that has at least aportion positioned between prongs 43. In this configuration, the lightbeam is directed to a spot on the substrate inside a region covered bythe magnetic field from the core. Consequently, the optical monitoringsystem 140 can measure the reflectivity of substantially the samelocation on the substrate as is being monitored by the eddy currentmonitoring system 40. Although not illustrated, core 42 and detector 146can be mounted on or attached to one or more printed circuit boards 160.

An example of a trace 250 generated by an optical monitoring system isshown in FIG. 11. The overall shape of intensity trace 250 may beexplained as follows. Initially, metal layer 16 has some initialtopography because of the topology of the underlying patterned layer 14.Due to this topography, the light beam scatters when it impinges themetal layer. As the polishing operation progresses in section 252 of thetrace, the metal layer becomes more planar and the reflectivity of thepolished metal layer increases. As the bulk of the metal layer isremoved in section 254 of the trace, the intensity remains relativelystable. Once the oxide layer begins to be exposed in the trace, theoverall signal strength drops quickly in section 256 of the trace. Oncethe oxide layer is entire exposed in the trace, the intensity stabilizesagain in section 258 of the trace, although it may undergo smalloscillations due to interferometric effects as the oxide layer isremoved.

Returning to FIGS. 3A, 3B and 4, the CMP apparatus 20 can also include aposition sensor 80, such as an optical interrupter, to sense when core42 and light source 44 are beneath substrate 10. For example, theoptical interrupter could be mounted at a fixed point opposite carrierhead 70. A flag 82 is attached to the periphery of the platen. The pointof attachment and length of flag 82 is selected so that it interruptsthe optical signal of sensor 80 while transparent section 36 sweepsbeneath substrate 10. Alternately, the CMP apparatus can include anencoder to determine the angular position of platen.

A general purpose programmable digital computer 90 receives theintensity signals and phase shift signals from the eddy current sensingsystem, and the intensity signals from the optical monitoring system.The printed circuit board 160 can include circuitry, such as a generalpurpose microprocessor or an application-specific integrated circuit, toconvert the signals from the eddy current sensing system and opticalmonitoring system into digital data. This digital data can be assembledinto discrete packets which are sent to computer 90 via a serialcommunication channel, e.g., RS-232. So long as both printed circuitboard 160 and computer 90 use the same packet format, computer 90 canextract and use the intensity and phase shift measurements in theendpoint or process control algorithm. For example, each packet caninclude five bytes, of which two bytes are optical signal data, twobytes are either amplitude or phase difference data for the eddy currentsignal, one bit indicates whether the packet includes amplitude or phaseshift data, and the remaining bits include flags for whether windowsection 36 is beneath the substrate, check-sum bits, and the like.

Since the monitoring systems sweep beneath the substrate with eachrotation of the platen, information on the metal layer thickness andexposure of the underlying layer is accumulated in-situ and on acontinuous real-time basis (once per platen rotation). The computer 90can be programmed to sample measurements from the monitoring system whenthe substrate generally overlies transparent section 36 (as determinedby the position sensor). As polishing progresses, the reflectivity orthickness of the metal layer changes, and the sampled signals vary withtime. The time varying sampled signals may be referred to as traces. Themeasurements from the monitoring systems can be displayed on an outputdevice 94 during polishing to permit the operator of the device tovisually monitor the progress of the polishing operation. In addition,as discussed below, the traces may be used to control the polishingprocess and determine the end-point of the metal layer polishingoperation.

In operation, CMP apparatus 20 uses eddy current monitoring system 40and optical monitoring system 140 to determine when the bulk of thefiller layer has been removed and to determine when the underlying stoplayer has been substantially exposed. The computer 90 applies processcontrol and endpoint detection logic to the sampled signals to determinewhen to change process parameter and to detect the polishing endpoint.Possible process control and endpoint criteria for the detector logicinclude local minima or maxima, changes in slope, threshold values inamplitude or slope, or combinations thereof.

In addition, computer 90 can be programmed to divide the measurementsfrom eddy current monitoring system 40 and optical monitoring system 140from each sweep beneath the substrate into a plurality of sampling zones96, to calculate the radial position of each sampling zone, to sort theamplitude measurements into radial ranges, to determine minimum, maximumand average measurements for each sampling zone, and to use multipleradial ranges to determine the polishing endpoint, as discussed in U.S.patent application Ser. No. 09/460,529, filed Dec. 13, 1999, theentirety of which is incorporated herein by reference.

Furthermore, computer 90 can be programmed to determine the thickness ofpolishing pad 30 and the absolute thickness of conductive layer 16 basedon the signals from eddy current monitoring system 40. In general, boththe intensity and phase shift signals from the eddy current detectordepends on the distance between core 40 and conductive layer 16. Inparticular, as shown in FIGS. 12A and 12B, as the polishing pad wearsand becomes thinner, conductive layer 16 will move closer to core 40,the coupling will increase, and consequently the strength of theamplitude and phase signals will decrease.

As previously noted, both the intensity and phase shift signals fromeddy current monitoring system 40 also depend on the thickness ofconductive layer 16. However, above a certain critical thickness of theconductive layer, the amplitude signal tends to be insensitive to thelayer thickness. Thus, when polishing begins, the amplitude signalremains constant until sufficient material has been removed (at time δ)that the conductive layer is thinner than the critical thickness. Atthis point, the amplitude signal begins to increase in strength. Incontrast, the phase shift signal reacts immediately to changes in thethickness of the conductive layer.

The intensity and phase shift signals can be used to determine thethickness of the polishing pad. Initially, a calibration step isperformed to polish a test substrate with a conductive layer thickerthan the critical thickness on two polishing pads of known, differentthicknesses. Alternatively, the calibration step could be performedusing the same pad at different stages of wear. During the calibrationstep, the strengths of the intensity signal and phase shift signal aremeasured for each polishing pad. From these measurements, twocoefficients Δ_(A) and Δ_(Φ) are calculated, representing the change insignal strength of the amplitude and phase shift signals, respectively,due to the pad thickness.

Thereafter, during polishing of a device wafer, the measured strengthsof the intensity and phase shift signals and the coefficients Δ_(A) andΔ₁₀₁ (or an equivalent lookup table) can be used to determine thethickness of the polishing pad. In particular, since the amplitudesignal is insensitive to the thickness of the conductive layer at thebeginning of the polishing process, this initial strength of theamplitude signal correlates to the thickness of the polishing pad. Themeasured thickness of the polishing pad may then be used to modify thepolishing parameters or generate an alert. For example, if the polishingpad thickness drops below a predetermined value, the computer cangenerate a signal to indicate that the polishing pad needs to bereplaced.

The intensity and phase shift signals can also be used to determine theabsolute thickness of the conductive layer on the substrate duringpolishing. Since the phase shift signal is immediately sensitive tochanges in the thickness of the conductive layer, a look-up table can begenerated (based on experimental measurements of a test substrate) torelate the strength of the phase shift signal to the thickness of theconductive layer. During polishing of a device substrate, the initialstrength of the amplitude signal can be measured at the beginning ofpolishing. Using the two coefficients Δ_(A) and Δ_(Φ), the computer cancalculate an adjusted phase signal strength that accounts for any offsetdue to changes in the polishing pad thickness. The computer can then usethe lookup table and the adjusted phase signal strength to accuratelycalculate the absolute thickness of the conductive layer.

Computer 90 may also be connected to the pressure mechanisms thatcontrol the pressure applied by carrier head 70, to carrier headrotation motor 76 to control the carrier head rotation rate, to theplaten rotation motor (not shown) to control the platen rotation rate,or to slurry distribution system 39 to control the slurry compositionsupplied to the polishing pad. Specifically, after sorting themeasurements into radial ranges, information on the metal film thicknesscan be fed in real-time into a closed-loop controller to periodically orcontinuously modify the polishing pressure profile applied by a carrierhead, as discussed in U.S. patent application Ser. No. 09/609,426, filedJul. 5, 2000, the entirety of which is incorporated herein by reference.For example, the computer could determine that the endpoint criteriahave been satisfied for the outer radial ranges but not for the innerradial ranges. This would indicate that the underlying layer has beenexposed in an annular outer area but not in an inner area of thesubstrate. In this case, the computer could reduce the diameter of thearea in which pressure is applied so that pressure is applied only tothe inner area of the substrate, thereby reducing dishing and erosion onthe outer area of the substrate.

The eddy current and optical monitoring systems can be used in a varietyof polishing systems. Either the polishing pad, or the carrier head, orboth can move to provide relative motion between the polishing surfaceand the substrate. The polishing pad can be a circular (or some othershape) pad secured to the platen, a tape extending between supply andtake-up rollers, or a continuous belt. The polishing pad can be affixedon a platen, incrementally advanced over a platen between polishingoperations, or driven continuously over the platen during polishing. Thepad can be secured to the platen during polishing, or there could be afluid bearing between the platen and polishing pad during polishing. Thepolishing pad can be a standard (e.g., polyurethane with or withoutfillers) rough pad, a soft pad, or a fixed-abrasive pad. Rather thantuning when the substrate is absent, the drive frequency of theoscillator can be tuned to a resonant frequency with a polished orunpolished substrate present (with or without the carrier head), or tosome other reference.

Although illustrated as positioned in the same hole, optical monitoringsystem 140 could be positioned at a different location on the platenthan eddy current monitoring system 40. For example, optical monitoringsystem 140 and eddy current monitoring system 40 could be positioned onopposite sides of the platen, so that they alternately scan thesubstrate surface.

Various aspects of the invention, such as placement of the coil on aside of the polishing surface opposite the substrate or the measurementof a phase difference, still apply if the eddy current sensor uses asingle coil. In a single coil system, both the oscillator and the sensecapacitor (and other sensor circuitry) are connected to the same coil.

The present invention has been described in terms of a preferredembodiment. The invention, however, is not limited to the embodimentdepicted and described. Rather, the scope of the invention is defined bythe appended claims.

1. An apparatus for chemical mechanical polishing, comprising: a platento support a polishing surface; an eddy current monitoring systempositioned in the platen to generate a first signal; an opticalmonitoring system positioned in the platen to generate a second signal;circuitry in the platen to combine the first signal from the eddycurrent monitoring system and second signals from the optical monitoringsystem into a third signal on an output line; and a computer to receivethe third signal on the output line and extract the first and secondsignals.
 2. The apparatus of claim 1, wherein the platen is rotatable.3. The apparatus of claim 2, further comprising a rotary electricalunion, and wherein the output line passes through the rotary electricalunion between the circuitry and the computer.
 4. The apparatus of claim1, further comprising a carrier head to hold a substrate in contact withthe polishing surface.
 5. The apparatus of claim 1, wherein thecircuitry assembles data from the first and second signals into packets,and the computer extracts the data from the packets.
 6. A chemicalmechanical polishing apparatus, comprising: a polishing surface; acarrier head to hold a substrate having a conductive layer disposedthereon in contact with the polishing surface; a motor to createrelative motion between the substrate and the polishing surface; an eddycurrent monitoring system including an inductor and a current source todrive the inductor to generate an alternating magnetic field thatinduces eddy currents in the conductive layer; a sensor to measure astrength of the magnetic field and a phase difference between themagnetic field and the drive signal; and a computer configured tocalculate a correction factor based on the strength of the magneticfield and calculate a thickness of the conductive layer from the phasedifference and the correction factor.
 7. The apparatus of claim 6,wherein the correction factor is indicative of a pad thickness.
 8. Theapparatus of claim 7, wherein the computer calculates the correctionfactor indicative of the pad thickness using the strength of themagnetic field corresponding to an initial thickness of the conductivelayer.
 9. An apparatus for chemical mechanical polishing, comprising: aplaten to support a polishing surface; a carrier head to hold asubstrate; an eddy current monitoring system to generate a first signalduring polishing, the eddy current monitoring system including a coilwrapped around a core to generate a magnetic field that extends to afirst region of the substrate, the first region including a conductivelayer having a thickness, and wherein the first signal is indicative ofthe thickness of the conductive layer in the first region; an opticalmonitoring system positioned to generate a second signal indicative ofthe thickness of the conductive layer in the first region duringpolishing, the optical monitoring system including a light source, thelight source positioned and oriented to direct a light beam to a spot inthe first region of the substrate so that the eddy current monitoringsystem and optical monitoring system measure substantially the samelocation on the substrate.
 10. An apparatus for chemical mechanicalpolishing, comprising: a polishing surface; a carrier head to hold asubstrate against the polishing surface; an eddy current monitoringsystem to generate a first signal during polishing, the eddy currentmonitoring system including an inductor to generate a magnetic fieldthat extends to a first region of the substrate, the first regionincluding a conductive layer having a thickness, and wherein the firstsignal is indicative of the thickness of the conductive layer in thefirst region; wherein the eddy current monitoring system includes a corehaving a plurality of prongs; and an optical monitoring systempositioned to generate a second signal indicative of the thickness ofthe conductive layer in the first region during polishing, the opticalmonitoring system including a light source, the light source positionedand oriented to direct a light beam to a spot in the first region of thesubstrate so that the eddy current monitoring system and opticalmonitoring system measure substantially the same location on thesubstrate, the optical monitoring system includes a detector positionedat least partially between the prongs.
 11. The apparatus of claim 10,wherein the light beam impinges the substrate at a point substantiallyequidistant from the prongs.
 12. An apparatus for chemical mechanicalpolishing, comprising: a polishing surface; a carrier head to hold asubstrate against the polishing surface; an eddy current monitoringsystem to generate a first signal during polishing, the eddy currentmonitoring system including an inductor to generate a magnetic fieldthat extends to a first region of the substrate, the first regionincluding a conductive layer having a thickness, and wherein the firstsignal is indicative of the thickness of the conductive layer in thefirst region, and wherein the eddy current monitoring system includes acore; and an optical monitoring system positioned to generate a secondsignal indicative of the thickness of the conductive layer duringpolishing, the optical monitoring system including a light source, thelight source positioned and oriented to direct a light beam to impingesthe substrate at a spot in the first region of the substrate directlyabove the core so that the eddy current monitoring system and opticalmonitoring system measure substantially the same location on thesubstrate.